Multilayer ceramic capacitor and manufacturing method of the same

ABSTRACT

A multilayer ceramic capacitor includes: a multilayer structure in which dielectric layers and internal electrode layers are alternately stacked and are alternately exposed to two edge faces of the multilayer structure; and a first cover layer and a second cover layer that sandwich the multilayer structure, a main component of the first cover layer and the second cover layer being the same as that of the dielectric layers, wherein the first cover layer is thicker than the second cover layer, wherein a concentration of Mn of at least a part of the first cover layer is higher than a concentration of Mn of the dielectric layers in an effective capacity region in which a set of internal electrode layers exposed to a first edge face of the multilayer structure face with another set of internal electrode layers exposed to a second edge face of the multilayer structure.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2018-028941, filed on Feb. 21,2018, the entire contents of which are incorporated herein by reference.

FIELD

A certain aspect of the present invention relates to a multilayerceramic capacitor and a manufacturing method of a multilayer ceramiccapacitor.

BACKGROUND

When an alternating voltage is applied to a multilayer ceramic capacitormounted on a substrate, extension and contraction caused byelectrostriction may occur in the multilayer ceramic capacitor. In thiscase, vibration caused by the extension and contraction is conducted tothe substrate, and acoustic noise phenomenon may occur. As a method forsuppressing the acoustic noise in the mounted multilayer ceramiccapacitor, there is disclosed a technology in which the acoustic noiseis suppressed by enlarging a thickness of a lower cover layer (on theside of the substrate) more than a thickness of an upper cover layer(for example, see Japanese Patent Application Publication No.2013-251522).

SUMMARY OF THE INVENTION

A densifying start temperature of the cover layer tends to be higherthan that of an effective capacity region. This is because contractionof an internal electrode does not have large influence on the coverlayer although densifying starts from a lower temperature in theeffective capacity region because of stress caused by the contraction ofthe internal electrode (for example, see Japanese Patent ApplicationPublication No. 2006-339285). Therefore, when the lower cover layer hasa large thickness for the purpose of suppressing the acoustic noise, adifference of densifying start temperatures between the effectivecapacity region and the lower cover layer gets larger. In this case, acrack may occur in an interface between the effective capacity regionand the cover layer. When proper sintering is achieved in the effectivecapacity region, sufficient sintering is not achieved in the cover layerand sufficient densifying is not achieved in the cover layer.Accordingly, reliability may be degraded.

The present invention has a purpose of providing a multilayer ceramiccapacitor that is capable of suppressing a defect of a case where one ofcover layers is thicker than the other and a manufacturing method of themultilayer ceramic capacitor.

According to an aspect of the present invention, there is provided amultilayer ceramic capacitor including: a multilayer structure having aparallelepiped shape in which each of a plurality of dielectric layersand each of a plurality of internal electrode layers are alternatelystacked and are alternately exposed to two edge faces of the multilayerstructure, a main component of the plurality of dielectric layers beinga ceramic; and a first cover layer and a second cover layer thatsandwich the multilayer structure in a stacking direction of themultilayer structure, a main component of the first cover layer and thesecond cover layer being the same as that of the dielectric layers,wherein the first cover layer is thicker than the second cover layer,wherein a concentration of Mn of at least a part of the first coverlayer is higher than a concentration of Mn of the dielectric layers inan effective capacity region in which a set of internal electrode layersexposed to a first edge face of the multilayer structure face withanother set of internal electrode layers exposed to a second edge faceof the multilayer structure.

According to another aspect of the present invention, there is provideda manufacturing method of a multilayer ceramic capacitor including: afirst step of forming a multilayer unit by providing a pattern of metalconductive paste on a green sheet including main component ceramicparticles; a second step of forming a multilayer ceramic structure bystacking a plurality of multilayer units obtained in the first step sothat a position of the pattern is alternately shifted; a third step ofproviding a first cover sheet and a second cover sheet so as to sandwichthe ceramic multilayer structure in a stacking direction, the firstcover sheet and the second cover sheet including main component ceramicparticles; and a fourth step of firing the multilayer ceramic structure,wherein the first cover sheet is thicker than the second cover sheet,wherein a concentration of Mn of at least a part of the first coversheet with respect to the main component ceramic is higher than aconcentration of Mn of the green sheet with respect to the maincomponent ceramic.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a partial perspective view of a multilayer ceramiccapacitor;

FIG. 2 illustrates a cross section taken along a line A-A of FIG. 1;

FIG. 3 illustrates a thickness of a first cover layer;

FIG. 4 illustrates densifying caused by sintering;

FIG. 5 illustrates a region in which a concentration of Mn is high;

FIG. 6 illustrates a manufacturing method of the multilayer ceramiccapacitor;

FIG. 7 illustrates results of examples and a comparative example; and

FIG. 8 illustrates results of examples and comparative examples.

DETAILED DESCRIPTION

A description will be given of an embodiment with reference to theaccompanying drawings.

Embodiment

FIG. 1 illustrates a partial perspective view of a multilayer ceramiccapacitor 100 in accordance with an embodiment. FIG. 2 illustrates across sectional view taken along a line A-A of FIG. 1. As illustrated inFIG. 1 and FIG. 2, the multilayer ceramic capacitor 100 includes amultilayer chip 10 having a rectangular parallelepiped shape, and a pairof external electrodes 20 a and 20 b that are respectively provided attwo edge faces of the multilayer chip 10 facing each other. In fourfaces other than the two edge faces of the multilayer chip 10, two facesother than an upper face and a lower face of the multilayer chip 10 in astacking direction are referred to as side faces. The externalelectrodes 20 a and 20 b extend to the upper face, the lower face andthe two side faces of the multilayer chip 10. However, the externalelectrodes 20 a and 20 b are spaced from each other.

The multilayer chip 10 has a structure designed to have dielectriclayers 11 and internal electrode layers 12 alternately stacked. Thedielectric layer 11 includes ceramic material acting as a dielectricmaterial. End edges of the internal electrode layers 12 are alternatelyexposed to a first edge face of the multilayer chip 10 and a second edgeface of the multilayer chip 10 that is different from the first edgeface. In the embodiment, the first edge face with the second edge face.The external electrode 20 a is provided on the first edge face. Theexternal electrode 20 b is provided on the second edge face. Thus, theinternal electrode layers 12 are alternately conducted to the externalelectrode 20 a and the external electrode 20 b.

In the multilayer structure of the dielectric layers 11 and the internalelectrode layers 12, two of the internal electrode layers 12 arerespectively stacked on outermost layers in the stacking direction. Afirst cover layer 13 a covers a lower face of the multilayer structure.A second cover layer 13 b covers an upper face of the multilayerstructure. Therefore, the first cover layer 13 a and the second coverlayer 13 b sandwich the multilayer structure in the stacking direction.A main component of the first cover layer 13 a and the second coverlayer 13 b is a ceramic material. For example, a main component of thefirst cover layer 13 a and the second cover layer 13 b is the same asthat of the dielectric layer 11.

For example, the multilayer ceramic capacitor 100 may have a length of1.0 mm, a width of 0.5 mm and a height of 0.5 mm. The multilayer ceramiccapacitor 100 may have a length of 1.0 mm, a width of 0.5 mm and aheight of 0.7 mm. The multilayer ceramic capacitor 100 may have a lengthof 1.0 mm, a width of 0.5 mm and a height of 0.9 mm. The multilayerceramic capacitor 100 may have a length of 1.6 mm, a width of 0.8 mm anda height of 0.8 mm. The multilayer ceramic capacitor 100 may have alength of 1.6 mm, a width of 0.8 mm and a height of 1.0 mm. Themultilayer ceramic capacitor 100 may have a length of 1.6 mm, a width of0.8 mm and a height of 1.2 mm. However, the size of the multilayerceramic capacitor 100 is not limited.

A main component of the internal electrode layers 12 is a base metalsuch as nickel (Ni), copper (Cu), tin (Sn) or the like. The internalelectrode layers 12 may be made of a noble metal such as platinum (Pt),palladium (Pd), silver (Ag), gold (Au) or alloy thereof. The dielectriclayers 11 are mainly composed of a ceramic material that is expressed bya general formula ABO₃ and has a perovskite structure. The perovskitestructure includes ABO₃-a having an off-stoichiometric composition. Forexample, the ceramic material is such as BaTiO₃ (barium titanate),CaZrO₃ (calcium zirconate), CaTiO₃ (calcium titanate), SrTiO₃ (strontiumtitanate), Ba_(1-x-y)Ca_(x)Sr_(y)Ti_(1-z)ZrzO₃ (0<x<1, 0<y<1, 0<z<1)having a perovskite structure.

As illustrated in FIG. 2, a region, in which a set of the internalelectrode layers 12 connected to the external electrode 20 a faceanother set of the internal electrode layers 12 connected to theexternal electrode 20 b, is a region generating electrical capacity inthe multilayer ceramic capacitor 100. And so, the region is referred toas an effective capacity region 14. That is, the effective capacityregion 14 is a region in which the internal electrode layers 12 next toeach other are connected to different external electrodes and face eachother.

A region, in which the internal electrode layers 12 connected to theexternal electrode 20 a face with each other without sandwiching theinternal electrode layer 12 connected to the external electrode 20 b, isreferred to as an end margin region 15. A region, in which the internalelectrode layers 12 connected to the external electrode 20 b face witheach other without sandwiching the internal electrode layer 12 connectedto the external electrode 20 a is another end margin region 15. That is,the end margin region 15 is a region in which a set of the internalelectrode layers 12 connected to one external electrode face with eachother without sandwiching the internal electrode layer 12 connected tothe other external electrode. The end margin region 15 is a region thatdoes not generate electrical capacity in the multilayer ceramiccapacitor 100.

The first cover layer 13 a is thicker than the second cover layer 13 b.In the structure, it is possible to suppress acoustic noise, when amount face of the multilayer ceramic capacitor 100 is the first coverlayer 13 a in a case where the multilayer ceramic capacitor 100 ismounted on a circuit substrate or the like. It is preferable that thefirst cover layer 13 a is thick, from a viewpoint of suppression of theacoustic noise. For example, when a thickness of the multilayer chip 10in the stacking direction is T and a thickness of the first cover layer13 a is Tc as illustrated in FIG. 3, it is preferable that Tc/T is 0.2or more. It is more preferable that Tc/T is 0.3 or more. It ispreferable that Tc is 50 μm or more. It is more preferable that Tc is100 μm or more. On the other hand, it is possible to suppress sizeenlargement of the multilayer ceramic capacitor 100, because the secondcover layer 13 b is thinner than the first cover layer 13 a.

A description will be given of an example of a manufacturing method ofthe multilayer chip 10. For example, it is possible to form thedielectric layers 11, the first cover layer 13 a and the second coverlayer 13 b by firing ceramic powders and densifying the ceramic powders.It is possible to form the internal electrode layers 12 by firing metalpowders and densifying the metal powders. The densifying of the ceramicpowders starts from a low temperature in a region corresponding to theeffective capacity region 14, because of stress caused by contraction ofthe metal powders. On the other hand, the densifying start temperatureof the ceramic powders gets higher in regions corresponding to the firstcover layer 13 a and the second cover layer 13 b, because the stresscaused by the contraction of the metal powders hardly influences on theregions. The densifying start temperature of the ceramic powdersspecifically gets higher in the region corresponding to the first coverlayer 13 a of which the thickness is large. Thus, a difference betweenthe densifying start temperatures gets larger. Therefore, a crack tendsto occur in an interface between the effective capacity region 14 andthe first cover layer 13 a. As illustrated in FIG. 4, when propersintering is achieved in the effective capacity region 14, the sinteringof the first cover layer 13 a is not sufficient and the densifying isnot sufficient. Therefore, the reliability of the multilayer ceramiccapacitor 100 may be degraded. Accordingly, although it is possible tosuppress the acoustic noise by enlarging the thickness of the firstcover layer 13 a, a defect may occur in the multilayer ceramic capacitor100.

And so, in the embodiment, a concentration of Mn (manganese) in at leasta part of the first cover layer 13 a is higher than a concentration ofMn in the dielectric layer 11 in the effective capacity region 14. Mnpromotes the densifying of the ceramic powders in the firing process andlowers the densifying start temperature. The densifying starttemperature of the ceramic powders in the region corresponding to thefirst cover layer 13 a is reduced and the difference of the densifyingstart temperatures between the region corresponding to the effectivecapacity region 14 and the region corresponding to the first cover layer13 a is reduced, when the concentration of Mn in the first cover layer13 a is enlarged. It is therefore possible to suppress the occurrence ofthe crack at in the interface between the effective capacity region 14and the first cover layer 13 a. It is possible to suppress thedegradation of the reliability of the multilayer ceramic capacitor 100,because the densifying of the first cover layer 13 a is promoted.Accordingly, it is possible to suppress the defect of a case where oneof the cover layers is thicker than the other.

It is possible to reduce the densifying start temperature by adding aglass component or the like to the first cover layer 13 a. However, inthis case, the glass component may cause grain growth. Therefore, whenthe glass component is used, it is difficult to suppress the graingrowth and reduce the densifying start temperature. On the other hand,Mn has a function of suppressing the grain growth. Therefore, in theembodiment, it is possible to suppress the grain growth of the firstcover layer 13 a and reduce the densifying start temperature.

When the concentration of Mn of the first cover layer 13 a isexcessively small, the densifying of the first cover layer 13 a may notbe necessarily and sufficiently promoted. And so, it is preferable thatthe concentration of Mn of the first cover layer 13 a has a lower limit.For example, it is preferable that the concentration of Mn of the firstcover layer 13 a is twice or more of the concentration of Mn of thedielectric layer 11 in the effective capacity region 14. It is morepreferable that the concentration of Mn of the first cover layer 13 a is5 times or more of the concentration of Mn of the dielectric layer 11 inthe effective capacity region 14. On the other hand, when theconcentration of Mn of the first cover layer 13 a is excessively high,the densifying start temperature may be reduced and a defect such as thecrack caused by a gap between the densifying start temperatures betweenthe effective capacity region 14 and the first cover layer 13 a mayoccur. And so, it is preferable that the concentration of Mn of thefirst cover layer 13 a has an upper limit. For example, it is preferablethat the concentration of Mn of the first cover layer 13 a is 30 timesor less of the concentration of Mn of the dielectric layer 11 in theeffective capacity region 14. It is more preferable that theconcentration of Mn of the first cover layer 13 a is 15 times or less ofthe concentration of Mn of the dielectric layer 11 in the effectivecapacity region 14.

A part of the first cover layer 13 a far from the effective capacityregion 14 is not subjected to influence of stress caused by contractionof metal powders. Therefore, the densifying start temperature of thepart may be specifically high. And so, as illustrated in FIG. 5, it ispreferable that a concentration of Mn in a region a of the first coverlayer 13 a spaced from the effective capacity region 14 by 50 μm or moreis higher than that of the dielectric layer 11 in the effective capacityregion 14. It is preferable that the concentration of Mn in the region ais twice or more and 30 times or less of the concentration of Mn of thedielectric layer 11 in the effective capacity region 14. It is morepreferable that the concentration of Mn in the region a is 5 times ormore and 15 times or less of the concentration of Mn of the dielectriclayer 11 in the effective capacity region 14.

Next, a description will be given of a manufacturing method of themultilayer ceramic capacitor 100. FIG. 6 illustrates a manufacturingmethod of the multilayer ceramic capacitor 100.

(Making Process of Raw Material Powder)

Ceramic powder for forming the dielectric layer 11 is prepared.Generally, an A site element and a B site element are included in thedielectric layer 11 in a sintered phase of grains of ABO₃. For example,BaTiO₃ is tetragonal compound having a perovskite structure and has ahigh dielectric constant. Generally, BaTiO₃ is obtained by reacting atitanium material such as titanium dioxide with a barium material suchas barium carbonate and synthesizing barium titanate. Various methodscan be used as a synthesizing method of the ceramic structuring thedielectric layer 11. For example, a solid-phase method, a sol-gelmethod, a hydrothermal method or the like can be used.

Additive compound may be added to the resulting ceramic powder inaccordance with purposes. The additive compound may be an oxide of Mg(magnesium), Mn, V (vanadium), Cr (chromium) or a rare earth element (Y(yttrium), Sm (samarium), Eu (europium), Gd (gadolinium), Tb (terbium),Dy (dysprosium), Ho (holmium), Er (erbium), Tm (thulium) and Yb(ytterbium)), or an oxide of Co (cobalt), Ni, Li (lithium), B (boron),Na (sodium), K (potassium) and Si (silicon), or glass.

In the embodiment, it is preferable that ceramic particles structuringthe dielectric layer 11 are mixed with compound including additives andare calcined in a temperature range from 820 degrees C. to 1150 degreesC. Next, the resulting ceramic particles are wet-blended with additivesand are dried and crushed. Thus, ceramic powder is obtained. Forexample, it is preferable that an average grain diameter of theresulting ceramic powders is 50 nm to 300 nm from a viewpoint ofthickness reduction of the dielectric layer 11. For example, the graindiameter may be adjusted by crushing the resulting ceramic powder asneeded. Alternatively, the grain diameter of the resulting ceramic powermay be adjusted by combining the crushing and classifying.

(Stacking process) Next, a binder such as polyvinyl butyral (PVB) resin,an organic solvent such as ethanol or toluene, and a plasticizer areadded to the resulting ceramic powder and wet-blended. With use of theresulting slurry, a stripe-shaped dielectric green sheet with athickness of 3 μm to 10 μm is coated on a base material by, for example,a die coater method or a doctor blade method, and then dried.

Then, a pattern of the internal electrode layer 12 is provided on thesurface of the dielectric green sheet by printing metal conductive pastefor forming an internal electrode with use of screen printing or gravureprinting. The conductive paste includes an organic binder. A pluralityof patterns are alternatively exposed to the pair of externalelectrodes. The metal conductive paste includes ceramic particles as aco-material. A main component of the ceramic particles is not limited.However, it is preferable that the main component of the ceramicparticles is the same as that of the dielectric layer 11. For example,BatiO₃ of which an average grain diameter is 50 nm or less may be evenlydispersed.

Then, the dielectric green sheet on which the internal electrode layerpattern is printed is stamped into a predetermined size, and apredetermined number (for example, 100 to 500) of stamped dielectricgreen sheets are stacked while the base material is peeled so that theinternal electrode layers 12 and the dielectric layers 11 are alternatedwith each other and the end edges of the internal electrode layers 12are alternately exposed to both edge faces in the length direction ofthe dielectric layer 11 so as to be alternately led out to a pair ofexternal electrodes of different polarizations.

A first cover sheet to be the first cover layer 13 a and a second coversheet to be the second cover layer 13 b are crimped so as to sandwichthe resulting ceramic multilayer structure in the stacking direction.The resulting structure is cut into a predetermined size (for example,1.0 mm×0.5 mm). In the embodiment, an added amount of Mn is adjusted sothat the concentration of Mn of at least a part of the first cover sheetwith respect to the main component ceramic is higher than that of thedielectric green sheet of the ceramic multilayer structure with respectto the main component ceramic. For example, the added amount of Mn maybe adjusted so that the concentration of Mn of a region of the firstcover sheet spaced from the ceramic multilayer structure by 50 μm ormore with respect to the main component ceramic is higher than that ofthe dielectric green sheet of the ceramic multilayer structure withrespect to the main component ceramic.

And, the thickness of the first cover sheet is larger than the thicknessof the second cover sheet. When the first cover sheet and the secondcover sheet are formed by stacking a plurality of dielectric greensheets, the number of the stacked dielectric green sheets in the firstcover sheet is larger than the number of the stacked dielectric greensheets in the second cover sheet. When the concentration of Mn of thefirst cover sheet has distribution, the added amount of Mn may beenlarged in a part of the dielectric green sheet in the region of whichthe concentration of Mn is high.

The binder is removed from the resulting compact in N₂ atmosphere. Afterthat, metal paste to be the base layers of the external electrodes 20 aand 20 b is coated from both edge faces of the compact to the side facesand is dried. The metal paste includes metal filler including a maincomponent metal of the external electrodes 20 a and 20 b, a co-material,a binder, a solvent and so on.

(Firing Process)

The binder is removed from the resulting compact in N₂ atmosphere of atemperature range of 250 degrees C. to 500 degrees C. After that, theresulting compact is fired for ten minutes to 2 hours in a reductiveatmosphere of which an oxygen partial pressure is 10⁻⁵ to 10⁻⁸ atm in atemperature range of 1100 degrees C. to 1300 degrees C. Thus, eachcompound is sintered and grown into grains.

(Re-Oxidizing Process)

After that, a re-oxidizing process may be performed in N₂ gas atmospherein a temperature range of 600 degrees C. to 1000 degrees C.

(Forming Process of External Electrode)

After that, with a plating process, Cu, Ni, Sn and so on may be coatedon a base layer of the external electrodes 20 a and 20 b. Thus, themultilayer ceramic capacitor 100 is manufactured.

In the manufacturing method of the embodiment, the concentration of Mnof at least a part of the first cover sheet with respect to the maincomponent ceramic is higher than that of the dielectric green sheet inthe ceramic multilayer structure with respect to the main componentceramic. In this case, the densifying start temperature of the ceramicpowder in the first cover sheet is reduced, and the difference of thedensifying start temperatures between the ceramic multilayer structureand the first cover sheet is reduced. It is therefore possible tosuppress the occurrence of the crack in the interface between theeffective capacity region 14 and the first cover layer 13 a in themultilayer ceramic capacitor 100. And it is possible to suppress thedegradation of the reliability of the multilayer ceramic capacitor 100because the densifying of the first cover layer 13 a is promoted. And itis possible to suppress the acoustic noise because the first cover layer13 a is thicker than the second cover layer 13 b. Accordingly, it ispossible to suppress the defect of the case where one of cover layers isthicker than the other.

EXAMPLES

The multilayer ceramic capacitors in accordance with the embodiment weremade. And, property of the multilayer ceramic capacitors was measured.

Comparative Example 1 and Examples 1 to 4

Ceramic powder of BaTiO₃ was prepared. Additives and sinteringassistants were added to the ceramic powder. The resulting ceramicpowder was sufficiently wet-blended and crushed with a ball mil. Thus,the dielectric material was obtained. An organic binder and a solventwere added to the dielectric material. And dielectric green sheets weremade by a doctor blade method. A thickness of the dielectric green sheetwas 0.8 μm. The organic binder was polyvinyl butyral (PVB) resin or thelike. The solvent was ethanol, toluene or the like. And a plasticizerand so on were added. Next, the conductive paste for forming theinternal electrode layer was formed by a planetary boll mill. Theconductive paste included a main component metal (Ni) powder of theinternal electrode layer 12, a co-material (barium titanate), a binder(ethyl cellulose), a solvent and an auxiliary as needed.

The conductive paste for forming the internal electrode layer wasscreen-printed on the dielectric green sheet. 250 of the dielectricgreen sheets on which the conductive paste for forming the internalelectrode layer was printed were stacked, and a first cover sheet wasstacked on the stacked dielectric green sheets. A second cover sheet wasstacked under the stacked dielectric green sheets. The first cover sheetand the second cover sheet were formed by stacking a plurality ofdielectric green sheets. The thicknesses of the first cover sheet andthe second cover sheet were adjusted by adjusting the number of thestacked dielectric green sheets.

After that, a ceramic multilayer structure was obtained by athermocompression bonding. And the ceramic multilayer structure was cutinto a predetermined size. The binder was removed from the ceramicmultilayer structure in N₂ atmosphere. After that, the metal paste to bethe base layers of the external electrodes 20 a and 20 b including themetal filler of which a main component was Ni, the co-material, thebinder and the solvent was coated from the both edge faces to the sidefaces of the ceramic multilayer structure and was dried. After that, theresulting multilayer structure was fired together with the metal pastefor 10 minutes to 2 hours in a reductive atmosphere in a temperaturerange of 1100 degrees C. to 1300 degrees C. And, a sintered structurewas formed.

The resulting sintered structure had a length of 1.0 mm and a width of0.5 mm. The sintered structure was subjected to a re-oxidation processat 800 degrees C. in N₂ atmosphere. After that, by a plating process, aCu-plated layer, a Ni-plated layer and a Sn-plated layer were formed ona surface of a base layer of the external electrodes 20 a and 20 b. And,the multilayer ceramic capacitor 100 was obtained.

As shown in Table 1, in the comparative example 1, the thickness T ofthe multilayer chip 10 in the stacking direction was 500 μm. Thethickness T in the example 1 was 590 μm. The thickness T in the example2 was 685 μm. The thickness T in the example 3 was 795 μm. The thicknessT in the example 4 was 950 μm. In the comparative example 1, thethickness Tc of the first cover layer 13 a was 25 μm. The thickness Tcin the example 1 was 115 μm. The thickness Tc in the example 2 was 210μm. The thickness Tc in the example 3 was 320 μm. The thickness Tc inthe example 4 was 475 μm. Therefore, Tc/T was 0.05 in the comparativeexample 1. Tc/T was 0.19 in the example 1. Tc/T was 0.31 in the example2. Tc/T was 0.40 in the example 3. Tc/T was 0.50 in the example 4. Inany one of the comparative example 1 and the examples 1 to 4, thethickness of the second cover layer 13 b was 25 μm, and the thickness ofthe effective capacity region 14 was 450 μm. The structure of thecomparative example 1 regulated by these thicknesses is referred to as astructure 1. The structure of the example 1 regulated by thesethicknesses is referred to as a structure 2. The structure of theexample 2 regulated by these thicknesses is referred to as a structure3. The structure of the example 3 regulated by these thicknesses isreferred to as a structure 4. The structure of the example 4 regulatedby these thicknesses is referred to as a structure 5.

TABLE 1 EFFECTIVE 2nd CAPACITY SOUND STRUCTURE Tc T COVER REGIONPRESSURE No. (μm) (μm) (μm) (μm) Tc/T (dB) COMPARATIVE 1 25 500 25 4500.05 30 EXAMPLE 1 EXAMPLE 1 2 115 590 25 450 0.19 24 EXMAPLE 2 3 210 68525 450 0.31 20 EXAMPLE 3 4 320 795 25 450 0.40 18 EXAMPLE 4 5 475 950 25450 0.50 16

10 samples were subjected to a sound pressure test, with respect to eachof the comparative example 1 and the examples 1 to 4. In the soundpressure test, each sample was mounted on a substrates so that the firstcover layer 13 a was on the substrate side. An alternating voltage of 5Vwas applied to the external electrodes 20 a and 20 b, and a frequency ofthe alternating voltage was increased from 0 MHz to 1 MHz. A soundpressure in an audible range (dB) generated during the applying of thevoltage was measured by TYPe-3560-B130 made by BRUEL KJAER JAPAN in asound-insulation and anechoic room made by YOKOHAMA SOUND ENVIRONMENTSYSTEMS. The measured sound pressure was an average of 10 measuredresults. As shown in Table 1, the sound pressure was a high value of 30dB, in the comparative example 1. It is thought that this was becausethe thickness Tc of the first cover layer 13 a was the same as that ofthe second cover layer 13 b, and the first cover layer 13 a was notthick. On the other hand, in the examples 1 to 4, the sound pressure wasa low value of less than 30 dB. It is thought that this was because thethickness Tc was larger than the thickness of the second cover layer 13b, and the first cover layer 13 a was thick. In particular, in theexamples 2 to 4, the sound pressure was a much lower value that wasequal to or less than 20 dB. It is thought that this was because Tc/Twas 0.20 or more and the first cover layer 13 a was sufficiently thick.

Comparative Examples 2 and 3 and Examples 5 to 10

In a comparative example 2, Mn was not added to the first cover layer 13a, in each of the structures 2 to 5. That is, in the comparative example2, in each of the structures 2 to 5, the concentration of Mn of thefirst cover layer 13 a was 0 times as that of the effective capacityregion 14. In the comparative examples 3, in each of the structures 2 to5, the concentration of Mn of the first cover layer 13 a was the same asthat of the effective capacity region 14. That is, in the comparativeexample 3, the concentration of Mn of the first cover layer 13 a was thesame as that of the effective capacity region 14. In the example 5, ineach of the structures 2 to 5, the concentration of Mn of the firstcover layer 13 a was twice as that of the effective capacity region 14.In the example 6, in each of the structures 2 to 5, the concentration ofMn of the first cover layer 13 a was 5 times as that of the effectivecapacity region 14. In the example 7, in each of the structures 2 to 5,the concentration of Mn of the first cover layer 13 a was 10 times asthat of the effective capacity region 14. In the example 8, in each ofthe structures 2 to 5, the concentration of Mn of the first cover layer13 a was 15 times as that of the effective capacity region 14. In theexample 9, in each of the structures 2 to 5, the concentration of Mn ofthe first cover layer 13 a was 20 times as that of the effectivecapacity region 14. In the example 10, in each of the structures 2 to 5,the concentration of Mn of the first cover layer 13 a was 30 times asthat of the effective capacity region 14. The ratio of the concentrationof Mn of the first cover layer 13 a and the concentration of Mn of theeffective capacity region 14 was adjusted with use of the added amountof Mn to the first cover sheet.

1000 samples were made with respect to each of the comparative examples2 and 3 and the examples 5 to 10. And a crack occurrence rate in theinterface between the first cover layer 13 a and the effective capacityregion 14 was measured. Table 2 shows the results. FIG. 7 illustratesthe results of the structure 3. As shown in Table 2 and FIG. 7, withrespect to the structure 3, the crack occurrence rate was 100% in thecomparative example 2. It is thought that this was because thedifference of the densifying start temperatures between the first coverlayer 13 a and the effective capacity region 14 was large because Mn wasnot added to the first cover sheet and the concentration of Mn of thefirst cover layer 13 a was zero times as that of the effective capacityregion 14. In the comparative example 3, the crack occurrence rate wasnot 100% but a large value of 5%. It is thought that this was becausethe difference of the densifying start temperatures between the firstcover layer 13 a and the effective capacity region 14 was notsufficiently reduced because the concentration of Mn of the first coverlayer 13 a was equal to that of the effective capacity region 14.

TABLE 2 Mn CONCENTRATION CRACK OCCURRENCE RATE (%) RATIO STRUCTURE 2STRUCTURE 3 STRUCTURE 4 STRUCTURE 5 COMPARATIVE 0 100 100 100 100EXAMPLE 2 COMPARATIVE 1 3 5 8 10 EXMAPLE 3 EXAMPLE 5 2 0.5 1 1.7 2.2EXAMPLE 6 5 0 0 0.2 0.5 EXAMPLE 7 10 0 0 0 0 EXAMPLE 8 15 0 0 0 0EXAMPLE 9 20 0.2 0.5 0.6 0.8 EXAMPLE 10 30 0.5 1 1.5 2

On the other hand, in the examples 5 to 10, the crack occurrence ratewas a low value of 1% or less. It is thought that this was because theconcentration of Mn of the first cover layer 13 a was higher than thatof the effective capacity region 14 and the difference of the densifyingstart temperatures between the first cover layer 13 a and the effectivecapacity region 14 was sufficiently small. In particular, in theexamples 6 to 9, the crack occurrence rate was further reduced. It isthought that this was because the concentration of Mn of the first coverlayer 13 a was 5 times to 20 times of that of the effective capacityregion 14 and the difference of the densifying start temperaturesbetween the first cover layer 13 a and the effective capacity region 14was sufficiently reduced.

As illustrated in Table 2 and FIG. 8, similar results were also observedin the structures 2, 4 and 5. That is, the crack occurrence rate waslowered when the concentration of Mn of the first cover layer 13 a washigher than that of the dielectric layer 11 in the effective capacityregion 14. And, the crack occurrence rate was further lowered when theconcentration of Mn of the first cover layer 13 a was 5 times to 20times as that of the effective capacity region 14.

Although the embodiments of the present invention have been described indetail, it is to be understood that the various change, substitutions,and alterations could be made hereto without departing from the spiritand scope of the invention.

What is claimed is:
 1. A multilayer ceramic capacitor comprising: amultilayer structure having a parallelepiped shape in which each of aplurality of dielectric layers and each of a plurality of internalelectrode layers are alternately stacked and are alternately exposed totwo edge faces of the multilayer structure, a main component of theplurality of dielectric layers being a ceramic; and a first cover layerand a second cover layer that sandwich the multilayer structure in astacking direction of the multilayer structure, a main component of thefirst cover layer and the second cover layer being the same as that ofthe dielectric layers, wherein the first cover layer is thicker than thesecond cover layer, wherein a concentration of Mn of at least a part ofthe first cover layer is higher than a concentration of Mn of thedielectric layers in an effective capacity region in which a set ofinternal electrode layers exposed to a first edge face of the multilayerstructure face with another set of internal electrode layers exposed toa second edge face of the multilayer structure.
 2. The multilayerceramic capacitor as claimed in claim 1, wherein the concentration of Mnin the at least a part of the first cover layer is twice or more and 30times or less of the concentration of Mn of the dielectric layers in theeffective capacity region.
 3. The multilayer ceramic capacitor asclaimed in claim 1, wherein Tc/T is 0.2 or more when a total thicknessof the multilayer structure, the first cover layer and the second coverlayer is T, and a thickness of the first cover layer is Tc.
 4. Themultilayer ceramic capacitor as claimed in claim 1, wherein a thicknessof the first cover layer is more than 50 μm.
 5. The multilayer ceramiccapacitor as claimed in claim 4, wherein the at least a part of thefirst cover layer is spaced from the multilayer structure by 50 μm ormore.
 6. A manufacturing method of a multilayer ceramic capacitorcomprising: a first step of forming a multilayer unit by providing apattern of metal conductive paste on a green sheet including maincomponent ceramic particles; a second step of forming a multilayerceramic structure by stacking a plurality of multilayer units obtainedin the first step so that a position of the pattern is alternatelyshifted; a third step of providing a first cover sheet and a secondcover sheet so as to sandwich the ceramic multilayer structure in astacking direction, the first cover sheet and the second cover sheetincluding main component ceramic particles; and a fourth step of firingthe multilayer ceramic structure, wherein the first cover sheet isthicker than the second cover sheet, wherein a concentration of Mn of atleast a part of the first cover sheet with respect to the main componentceramic is higher than a concentration of Mn of the green sheet withrespect to the main component ceramic.